There is always a need for precise and reliable metrology to monitor the properties of thin films, especially in the semiconductor and magnetic head industries. Thin film properties of interest include the thickness of one or more layers, the surface roughness, the interface roughness between different layers, the optical properties of the different layers, the compositional properties of the different layers and the compositional uniformity of the film stack. Ellipsometry is particularly well suited to this task when the thickness is less than 100 nm, when there are more than two layers present or when there are compositional variations. Additionally, dimensional measurements such as linewidth, sidewall angle, and height can be extracted using ellipsometry and/or reflectometry.
An ellipsometer is a measurement tool used to determine the change in polarization state of an electromagnetic wave after interaction with a sample. The determination of this polarization state can yield information about the thin film properties such as those listed above. In general, an ellipsometer is a polarization-state-in, polarization-state-out device. FIG. 1 shows a simple block diagram of a typical ellipsometer 10, which includes a Polarization State Generator (PSG) 12 that generates an electromagnetic wave of a known polarization state and a Polarization State Detector (PSD) 16 that determines the polarization state of the electromagnetic wave after interaction with a sample 14. In FIG. 1 the interaction is shown in reflection mode, but it should be understood that the interaction may be in transmission mode, i.e., the PSD determines the polarization state of the electromagnetic wave after transmission through a sample. Different kinds of PSG/PSD configurations have been proposed and developed for ellipsometers. The advantages of each configuration are specific to the kind of extracted information that is desired.
The use of pulsed light source in metrology devices offers many advantages over conventional continuous light sources, as discussed in U.S. Pat. No. 6,002,477 to Hammer. A pulsed light source enables energization of the light source to be confined to the time over which a measurement is to be made, thereby reducing power consumption and very significantly extending the life of the light source.
In the thin film metrology field, the most popular ellipsometry configurations include a rotating polarizing element in the PSG and/or the PSD. Unfortunately, rotating polarizing elements cannot be used with pulsed light sources such as flash bulbs or pulsed lasers. When a rotating polarizing element is used with a pulsed light source, synchronization problems occur, resulting in inaccurate information being extracted. Furthermore, the light source intensity must be very constant over a whole optical rotation when using rotating elements, which is not possible with a pulsed light source, where the light source intensity varies significantly from pulse to pulse. This problem is aggravated for spectroscopic ellipsometry, where usually a multi-channel detector is utilized to record the whole spectrum. Such a photodiode array generally has a minimum reading time, which makes the use of a pulsed source in conjunction with a rotating element impossible.
A different kind of ellipsometer that has been extensively developed and used for thin film metrology and that does not have a rotating element is the photoelastic modulator ellipsometer (PME). This instrument employs a photoelastic modulator (PM) to change the polarization state of the light as a function of time either before or after reflection from the sample surface. FIG. 2 is a block diagram of a conventional PME 20. The PSG portion 21 of the PME 20 includes a light source 22 and a linear polarizer 24. The light source 22 generates a collimated beam (monochromatic or broadband radiation) that is transmitted through the linear polarizer 24. The linearly polarized beam is reflected from the sample surface 26 thereby modifying the polarization state of the electromagnetic beam. The PSD portion 27 of the PME 20 includes a PM (or Pockels cell) 28, another linear polarizer 30, and a detector 32.
Unfortunately, photoelastic modulators and Pockels cells introduce a time dependent phase that creates synchronization problems when used with a pulsed light source, similar to those seen in ellipsometers utilizing a rotating polarizing element. Thus, a pulsed light source is impractical in conjunction with an ellipsometer configuration that utilizes a photoelastic modulator or Pockels cell.
What is needed is an ellipsometer configuration (monochromatic or spectroscopic) that does not use moving parts or a phase modulator, i.e., a configuration that is time-independent so that it can be used with a pulsed light source, with the advantage over a continuous light source being that a pulsed light source generates less heat, is more intense and has a longer lifetime. Moreover, such an ellipsometer can be compact and robust, minimizing cost and maintenance. Such a configuration will be particularly suitable for integration into existing process tools due to its reduced size.